Many factors can affect the quality of photographic prints made from digital images. In particular, the noise characteristics of a digital image can have a serious impact on overall photographic print quality. For example, most digital image processing systems that manipulate the pixel data of a digital image prior to printing include a stage of spatial sharpening, i.e. enhancing the spatial detail in the image. For digital images that contain an excessive level of noise, this spatial sharpening operation can degrade the overall photographic print quality due to the amplification of the noise.
In general, it is difficult to determine the level of noise in a digital image from using only the pixel values of the digital image. In commonly assigned U.S. Pat. No. 5,923,775, Snyder et al. disclose a method of image processing which includes a step of estimating the noise characteristics of a digital image and using the estimates of the noise characteristics in conjunction with a noise removal system to reduce the amount of noise in the digital image. The method disclosed by Snyder et al. is designed to work for digital images derived from radiographic film and includes a multiple step process for the noise characteristics estimation procedure. This method is computationally complex and can fail for some digital images due to the difficulty of automatically separating image structure (scene content) from noise structure.
In commonly assigned U.S. Pat. No. 6,038,011 Ikenoue, et al. disclose a system for recording non-image information on a photographic film, reading the non-image information along with the image information, and using the non-image information to vary the appearance of photographic prints made from the image information read from the photographic film. The method disclosed by Ikenoue, et al. includes the steps of memorizing conversion tables for converting photographic data into coded data that can be used in a printer, a conversion table being prepared for each of the recording formats of the photographic data recorded on a recording medium such as photographic film; recording index data along with the photographic data, the index data being indicative of the recording format of the photographic data; reading the index data and the photographic data of an image frame when printing the image frame; selecting one of the conversion tables corresponding to the index data; converting the photographic data read according to the one of the conversion tables selected; obtaining photometric values from the image frame; and determining the print-exposure amount based on the converted photographic data and the photometric values. This method claims to improve the appearance of the photographic prints by use of the recorded index data that indicates the type of recording format. While the method disclosed by Ikenoue, et al. may improve the print quality in color and tone characteristics, it does not improve the spatial detail of the photographic prints.
In commonly assigned U.S. Pat. No. 5,917,578 Nakamura discloses an apparatus and method for a photographic process in which information regarding camera, lens, and film used in photographing are recorded on the film. According to Nakamura's method, the information recorded on the film is used to determine the state of deterioration in quality of a state of worsening of graininess of the image. Based on this information, Nakamura's method corrects the worsening of graininess of the image. Although Nakamura discloses a general framework that includes recording and using non-image information to improve the quality of a photographic print, Nakamura fails to disclose any practical means or method with which to achieve improved image quality results. Furthermore, Nakamura's disclosure does not include any details with regard to implementation of a method for a digital imaging system. Therefore, Nakamura's method cannot be implemented in a practical digital imaging system.
It is known to measure the noise characteristics of photographic film as scanned with a particular film scanner. It is also well known in the field of photography that the photographic speed, as indicated by the ISO speed (or equivalently the EI exposure index), is a useful indication as to the expected magnitude of film grain noise associated with photographic film products. In general, photographic films rated with higher ISO speeds have more noise than photographic films with lower ISO speeds. It is known in the art that the noise characteristics of photographic films can be measured and used to improve the appearance of the digital images produced by scanning a photographic film strip and using the measured noise characteristics to control a noise reduction filter. It is also known in the art that the measured noise characteristics for a given ISO speed can be generalized for many photographic film products that have either the same or similar ISO speed rating. Thus the ISO speed rating of the photographic film product can be used to select a measured noise characteristic from a database of measured noise characteristics that approximates the noise magnitude of the photographic film without having to measure the noise magnitude of the photographic film product.
It would be desirable to apply a similar technique to the processing of digital images captured by a digital camera. Unfortunately, the noise characteristics of digital cameras are not as consistent with respect to ISO speed as those of photographic film. Factors contributing to the inconsistency include: different noise characteristics between professional and consumer grade digital cameras having the same rated ISO speed; compression modes; image resolution (i.e. the number of pixels in the digital image); contrast settings; in camera spatial filtering operations (including sharpening, noise cleaning and color filter array processing); color white balance processing; and lighting mode (flash, tungsten, fluorescent, daylight). Many of these modes and settings that affect noise characteristics are user selectable.
There is a need therefore for an improved method of enhancing digital images produced by digital cameras.